Coles, a Harvard professor of psychiatry and social ethics and the Pulitzer Prize-winning author of Children of Crisis, is one of those rare writers who can gracefully combine intellectual rigor with the idiom of spiritual quest. Here, despite the title, Coles is concerned not just with the secular mind--the mind that exists and exerts its will in the contentious world--but with the blurry intersection of the secular and the sacred. ""I try,"" he writes, ""to explore this matter of two minds, secular thinking and its constant search for moral, if not spiritual, sanction."" Over the years, Coles has interviewed notable theologians, psychiatrists, novelists and poets, juxtaposing their clinical and aesthetic takes on the psychology of the human soul with experiences of ordinary people. He discusses the writings of Freud, Walker Percy, Dorothy Day, William Carlos Williams and Kierkegaard, among others. Throughout the book, Coles meditates on the paradox that it is as a scientist that he approaches questions traditionally deemed religious or spiritual. It's a paradox that, as Coles notes, manifests in many ways: Freud, who despised religion, became an object of ""secular idolatry"" as his work displaced the interior spiritual world that ""had been the territory of religion."" Coles also includes the not-at-all-famous, such as an Italian immigrant woman who realized the high cost of her Americanization: ""When I prayed to God, I used to talk to Him, now I talk to myself."" Finally, Coles assesses the impact of technology, including the possibility that science, in the form of neurology or genetics, may discern--and eventually mediate with drugs--human qualities, such as ""goodness"" or ""badness,"" that once were in the realm of the spiritual. The brevity and conversational style of the book is deceptive; this is a potent and powerful work readers will think about and return to again and again. (Apr.)